1. Field of the Invention
The present invention is directed to printing a pattern, such as an image or other indicia, onto a surface, and more specifically to printing a pattern onto a surface utilizing at least one microelectromechanical system (MEMS) actuator. The present invention in exemplary form makes use of Joule heating to actuate a beam that is capable of displacing ink from a chamber and onto a surface of a print medium.
2. Background of the Invention
There are two basic types of microelectromechanical system (MEMS) actuators: single material actuators and composite material actuators. Both types of actuators are based upon the principle of Joule heating to thermally expand a micromachined material to generate the requisite displacement.
Referencing FIG. 14, the well-known Guckel actuator is an example of a single material MEMS actuator 10. The actuator 10 may be micromachined from silicon or polysilicon and when a voltage is applied at the anchored end of the device, the thin arm 12 has a much higher current density than the wide arm 14. The thin arm 12 becomes elevated in temperature to a greater degree than the wide arm 14 as a result of the current density and thus, the thin arm 14 will tend expand more than the wide arm 16. The result is differential expansion between the thin arm 14 and wide arm 16 providing a net movement toward the wide arm 16.
Exemplary single material actuators have been reported as comprising a 1575 Ohm actuator, 2200 microns long, with thin/wide arms being 40/255 microns wide, respectively (University of Pennsylvania, NSF Grant DMI-97-33196). When 9 volts was applied across this single material actuator, Joule heating caused an average temperature rise of approximately 230° C. The temperature difference between the thin and wide arms was approximately 50° C. and the differential thermal expansion produced a net deflection or movement of about 8 microns.
Another example of a single material MEMS actuator is disclosed in a NSF Grant ECS-9734421 (University of California at Berkeley). In this example, the actuator is micromachined from polysilicon and has dimensions of 2×2×100 microns, which each end of the actuator being mounted to an anchor point. Thermal expansion of the polysilicon causes the beam to buckle as the expansion is constrained at the ends of the beam by the anchor points. The authors reported that a continuous current of 4.2 mA through the beam caused a steady state ΔT of 900° C., resulting in a deflection of 3 microns.
In contrast to the single material examples, composite material actuators may use a beam structure consisting of two different materials having two different thermal expansion coefficients. Joule heating is used to raise the temperature of the beam and, because the two materials have different thermal expansion coefficients, a net movement in one or more directions results.